1. Field of the Invention
This invention relates to a polyolefin resin composition usable mainly for processing of sheet.
2. Description of the Prior Art
When polyolefin resin sheets are subjected to vacuum forming or air pressure forming, olefin homopolymers or copolymers exhibit a heat-sensitive behavior at their thermal deformation temperatures or higher, particularly in the vicinity of their softening temperatures, and in particular, their melt viscosities suddenly drop within a very narrow temperature range. In the processing of sheet, generally a sudden change of viscosity in the vicinity of the processing temperature makes the forming properties inferior and also makes a uniform forming fabrication impossible. Thus, use of resins having different melting points in admixture has been tried for improving the hot processing characteristics of resins, resulting, however, in degrading the compatibilities of resins with each other. For example, in the case of polypropylene-polyethylene blends, it is possible to improve their compatibilities by replacing polypropylene by a propylene-ethylene copolymer or adding a synthetic rubber composed mainly of an ethylene-propylene copolymer. However, even these blends have such problems that their melt viscosities are not always high and their flowability cannot be sufficiently improved.
On the other hand, with regard to resin compositions proposed for solving these problems, Japanese patent publication No. Sho 46-43990/1971 discloses a resin composition consisting of a graft-modified polypropylene and a granular cellulosic high molecular weight substance. However, such a composition has drawbacks in that it is inferior in the resin processing temperature and deficient in the processing stability at the time of forming shaped articles.
Further, Japanese patent application laid-open No. Sho 56-34737/1981 discloses a composition consisting of a polypropylene block copolymer, a high density polyethylene, a plasticizer, a petroleum resin and vegetable fibers, but vacuum forming properties are not always satisfactory.
Furthermore Japanese patent application laid-open No. Sho 55-25495/1980 discloses a composition consisting of a high density polyethylene modified with maleic anhydride and cellulosic fibers, but such a composition is also inferior in the heat-resistance and the vacuum forming properties.
The object of the present invention is to solve these problems and provide a polyolefin resin composition which is suitable mainly for affording superior processing properties of sheet and little in the variation of flow characteristics at hot processing temperatures.